It has been known in the prior art that unsaturated hydrocarbons may be obtained from the dehydrogenation of dehydrogenatable hydrocarbons. The dehydrogenation may be effected by subjecting the dehydrogenatable hydrocarbons to a dehydrogenation process at dehydrogenation conditions in the presence of certain catalytic compositions of matter which possess the ability to dehydrogenate said compounds with the resultant formation of olefinic hydrocarbons. The particular dehydrogenation catalysts which are employed are well known in the art and comprise such compounds as nickel composited on a solid support such as diatomaceous earth, kieselguhr, charcoal and iron composited on the same supports, etc.
Other dehydrogenation processes have employed, in addition to the dehydrogenation catalysts, an oxidation catalyst in the reaction process. The presence of the oxidation catalyst is necessitated by the fact that it is advantageous to oxidize the hydrogen which is produced by contact with an oxygen-containing gas in order to maintain the desired reaction temperature. For example, styrene, which is an important chemical compound utilized for the preparation of polystyrene, plastics, resins or synthetic elastomers such as styrene-butadiene rubber, etc. may be prepared from the dehydrogenation of ethylbenzene. The dehydrogenation of ethylbenzene into styrene, which is effected by treating ethylbenzene with steam in the presence of a modified iron catalyst, is endothermic in nature. The heat of reaction is about 30 Kcal per mole of ethylbenzene. Therefore, the temperature of the catalyst bed decreases significantly during the progress of the reaction in a commercial adiabatic reactor resulting in limitation of ethylbenzene conversion to a low level. The limitation of conversion arises from the fact that the equilibrium conversion of ethylbenzene is lowered and the rate of ethylbenzene dehydrogenation decreases as the reaction temperature decreases. The decrease of temperature adversely affects not only the conversion level, but also the selectivity for styrene since at equilibrium conditions, only undesirable side reactions continue to take place. Therefore, it is necessary to maintain the desired temperature level in order to provide a high equilibrium conversion level and a high reaction rate. In the conventional processes, the maintenance of temperature is attained by reheating the product stream with the addition of superheated steam between dehydrogenation catalyst beds using a multicatalyst bed reactor system. However, consumption of the additional superheated steam is considerably high and makes the dehydrogenation process costly. Accordingly, significant process economic improvements over the conventional ethylbenzene dehydrogenation processes can be achieved if the reaction temperature is somehow maintained while eliminating or reducing the additional superheated steam. One method of providing for the maintenance of the reaction temperature is to introduce oxygen into the reaction mixture by way of oxygen or an oxygen-containing gas such as air which will burn the hydrogen formed during the reaction, this combustion resulting in an exothermic reaction which will provide the necessary amount of heat and, in addition, will shift the equilibrium toward production of styrene since the hydrogen formed in the dehydrogenation is consumed. Consequently, a higher conversion and higher styrene selectivity are achievable.
The selective combustion of hydrogen with the oxygen in the oxygen-containing gas requires the presence of an oxidiation catalyst. There are some key requirements for the oxidation catalyst to be usable for such a purpose. The most important catalytic properly required is good catalytic stability since the oxidation catalyst must survive under very severe reaction conditions, namely at about 600.degree. to 650.degree. C. in the presence of steam. Under such conditions, porous inorganic materials such as aluminas, silicas and zeolites cannot maintain their pore structures for a long period of time, resulting in the permanent damage of catalysts prepared using such materials as supports, e.g. platinum supported on a porous high surface area alumina, silica, or zeolite. Secondly, the oxidation catalyst must be very active to achieve complete conversion of oxygen to avoid poisoning of iron-based dehydrogenation catalysts which are sensitively oxidized with oxygen to lose their dehydrogenation activities. Thirdly, the oxidation catalyst must be selective for oxidation of hydrogen. Otherwise, ethylbenzene and styrene are consumed to lower the efficiency of styrene production.
Various U.S. patents have described types of oxidation catalysts which may be employed in this process. For example, U.S. Pat. No. 3,437,703 describes a catalytic dehydrogenation process which employs, as a dehydrogenation catalyst, a composition known in the trade as Shell-105 which consists of from 87% to 90% ferric oxide, 2% to 3% chromium oxide and from 8% to 10% of potassium oxide. In addition, another dehydrogenation catalyst which is employed comprises a mixture of nickel, calcium, chromic oxide, graphite with a major portion of a phosphate radical. In addition to these dehydrogenation catalysts, the reaction also employs a catalyst for the oxidation step of the process comprising platinum or palladium in elemental form or as a soluble salt. Another U.S. patent, namely, U.S. Pat. No. 3,380,931, also discloses an oxidation catalyst which may be used in the oxidative dehydrogenation of compounds such as ethylbenzene to form styrene comprising an oxide of bismuth and an oxide of a metal of Group VIB of the Periodic Table such as molybdenum oxide, tungsten oxide or chromium oxide. In addition, the patent also states that minor amounts of arsenic may also be present in the catalytic composite as well as other metals or metalloids such as lead, silver, tin, manganese, phosphorus, silicon, boron and sulfur.
U.S. Pat. No. 3,855,330 discloses a method for the production of styrene in which ethylbenzene is treated in the vapor state by passage over a dehydrogenation catalyst and an oxidation catalyst while introducing oxygen into the reaction medium. The dehydrogenation catalysts which are employed are those which have been set forth in various prior U.S. patents and which may be similar in nature to the dehydrogenation catalysts previously discussed. The types of oxidation catalysts which may be employed will include platinum or palladium catalysts which are composited on alumina or molecular sieves of the zeolite type which have been charged with ferrous, heavy or noble metals. The patent lists the types of catalysts which are employed including copper or various zeolites, platinum on alumina, platinum on spinel, platinum and sodium on zeolites, platinum, sodium, and potassium on zeolites, etc.
In a process for the dehydrogenation of dehydrogenatable hydrocarbons wherein said hydrocarbons are treated with steam and a dehydrogenation catalyst along with a subsequent or concurrent treatment with an oxygen-containing gas in the presence of an oxidation catalyst, it will hereinafter be shown that by utilizing a catalyst of the type of the present invention, it is possible to obtain the desired product in an excellent yield with a concomitant use of the catalyst for a longer period of time due to the excellent stability of the catalyst during the reaction period.